RTF is a global modification that replaces all towns in the game.

No, no, no, here the spirit will be an outcast; it was the culprit of some cataclysm (in Dark Messiah, you can find a mention of a cataclysm that occurred 2000 years before the era of the Seventh Dragon). The spirit is imprisoned in a sword so that it doesn't cause even worse things.
In Heroes 5, Letos was a pirate, and his ships were destroyed by orcs in the second mission of the Orcs Campaign, so the plot will start with Letos stopping on an island, in the ancient city of dark dwarf-pirates. There, he will take an order to survey ancient ruins, where he will find this sword, but he will be met by an army of golems and elementals, and Letos will make a deal with the spirit – salvation in exchange for release from the blade.
Then, Letos will be teleported to the Wolf Duchy, where, in the ruins of a demon-worshipping temple, he will meet the resurrected Kaspar, and together they will go to Kartal.
Then there will be a coup – either Letos will seize power over the Chimera mages, or he will help the ruler's youngest son take the throne (but he must be insane, in the best traditions of the Chimera mages). This is how they will gain power.
The spirit in the sword will no longer be able to help them and will be freed. It can look like anything – a lich, a vampire, a ghost bound to armor, an elf, or it can be placed in a living creature (some Chimera general)... It will call itself the Ice King and will go in search of its destiny in the east.
The plot diverges.
1) Letos repels Tar-Ladd.
2) The Ice King meets Sareth and Zana in Stonehelm.
The first branch is short, and the second will have many events.

The second branch deviates from the lore, and in Dark Messiah, it's not entirely clear; you can get rid of Zana, you can simply kill the succubus (if you save her from the Necropolis lair and don't banish Zana), or as a lich (if she is killed there).
And we don't know much about Raileg, so after he goes on a quest and his unsuccessful attempt in one of the free cities, you can do anything with him.

Added 3 minutes ago
There isn't much known about the elementals either; you can take Igness from Heroes 3 (or another fire spirit, but it's easier with her; you can repaint the witch or dryad model in lava color, add some fire, and put it on the map) and build on that. Let's say she was summoned by other spirits or Askh herself to protect Ashan (in Heroes 6, there was a conversation between Namtara and Kirill, where Namtara expressed her fears and concerns for her children and the world in general).

Added 55 seconds ago
By the way, about the Gorgons, if they don't have enough space, they can be added as neutrals.

Added 4 minutes ago
I just had an idea, it's too crazy, but I'll share it.
We have creatures from the "old world." If Letos's acquaintances from the "old world" are hunting him, we can leave their models, give them creatures from the old castles, which will be like neutrals (as far as I know, you can add as many neutrals as you want). And Letos himself can be given dark elves mixed with dwarves in the early stages.

Added 3 minutes ago
As I understand it, the campaign for Letos will be the first, but we need more campaigns (at least three, like in the expansions).
If we develop the Igness option, this will be the second; we can also arrange a campaign for the Nagas before the campaign with Letos, who, while waging their own war (or just out of spite), destroy Letos's ship